Divalent counterion

Micellization depends upon a balance of forces and the cmc decreases with increasing hydrophobicity of the apolar groups, and for ionic amphiphiles also depends on the nature and concentration of counterions in solution. Added electrolytes decrease the cmc, and the effect increases with decreasing charge density of the counterion. Divalent counterions, however, lead to... 

Figure 4.13 Competitive adsorption by divalent counterions against monovalent counterions, (a) Partitioning of monovalent counterions, divalent counterions, and monovalent coions inside the counterion worm as a function of the concentration of the /AV2-tyP scilh (b) The net charge fraction of the polymer. (From Liu, S. et ah, J. Chem. P/iys., 119, 1813, 2003. With permission.)...

The effect is more than just a matter of pH. As shown in Fig. XV-14, phospholipid monolayers can be expanded at low pH values by the presence of phosphotungstate ions [123], which disrupt the stmctival order in the lipid film [124]. Uranyl ions, by contrast, contract the low-pH expanded phase presumably because of a type of counterion condensation [123]. These effects caution against using these ions as stains in electron microscopy. Clearly the nature of the counterion is very important. It is dramatically so with fatty acids that form an insoluble salt with the ion here quite low concentrations (10 M) of divalent ions lead to the formation of the metal salt unless the pH is quite low. Such films are much more condensed than the fatty-acid monolayers themselves [125-127]. 

Monovalent cations are compatible with CMC and have Httle effect on solution properties when added in moderate amounts. An exception is sUver ion, which precipitates CMC. Divalent cations show borderline behavior and trivalent cations form insoluble salts or gels. The effects vary with the specific cation and counterion, pH, DS, and manner in which the CMC and salt are brought into contact. High DS (0.9—1.2) CMCs are more tolerant of monovalent salts than lower DS types, and CMC in solution tolerates higher quantities of added salt than dry CMC added to a brine solution. 

Pyrazole and its C-methyl derivatives acting as 2-monohaptopyrazoles in a neutral or slightly acidic medium give M(HPz) X, complexes where M is a transition metal, X is the counterion and m is the valence of the transition metal, usually 2. The number of pyrazole molecules, n, for a given metal depends on the nature of X and on the steric effects of the pyrazole substituents, especially those at position 3. Complexes of 3(5)-methylpyrazole with salts of a number of divalent metals involve the less hindered tautomer, the 5-methylpyrazole (209). With pyrazole and 4- or 5-monosubstituted pyrazoles M(HPz)6X2... 

The type of counterion present in an ionomer may, or may not, have a significant effect of properties. For polyethylene-based ionomers, where the presence of crystallinity has an appreciable effect on properties, the type of counterion present does not appear to have a significant effect on either modulus or tensile strength, as Fig. 4 indicates. However, in amorphous ionomers, the effects of changing the counterion from a monovalent one, as in Na or K, to a divalent one, such as Ca, may be appreciable. 

As one example, in thin films of Na or K salts of PS-based ionomers cast from a nonpolar solvent, THF, shear deformation is only present when the ion content is near to or above the critical ion content of about 6 mol% and the TEM scan of Fig. 3, for a sample of 8.2 mol% demonstrates this but, for a THF-cast sample of a divalent Ca-salt of an SPS ionomer, having only an ion content of 4.1 mol%, both shear deformation zones and crazes are developed upon tensile straining in contrast to only crazing for the monovalent K-salt. This is evident from the TEM scans of Fig. 5. For the Ca-salt, one sees both an unfibrillated shear deformation zone, and, within this zone, a typical fibrillated craze. The Ca-salt also develops a much more extended rubbery plateau region than Na or K salts in storage modulus versus temperature curves and this is another indication that a stronger and more stable ionic network is present when divalent ions replace monovalent ones. Still another indication that the presence of divalent counterions can enhance mechanical properties comes from... 

The combined effects of a divalent Ca counterion and thermal treatment can be seen from studies of PMMA-based ionomers [16]. In thin films of Ca-salts of this ionomer cast from methylene chloride, and having an ion content of only 0.8 mol%, the only observed deformation was a series of long, localized crazes, similar to those seen in the PMMA homopolymer. When the ionomer samples were subject to an additional heat treatment (8 h at 100°C), the induced crazes were shorter in length and shear deformation zones were present. This behavior implies that the heat treatment enhanced the formation of ionic aggregates and increased the entanglement strand density. The deformation pattern attained is rather similar to that of Na salts having an ion content of about 6 mol% hence, substitution of divalent Ca for monovalent Na permits comparable deformation modes, including some shear, to be obtained at much lower ion contents. 

Mechanisms of micellar reactions have been studied by a kinetic study of the state of the proton at the surface of dodecyl sulfate micelles [191]. Surface diffusion constants of Ni(II) on a sodium dodecyl sulfate micelle were studied by electron spin resonance (ESR). The lateral diffusion constant of Ni(II) was found to be three orders of magnitude less than that in ordinary aqueous solutions [192]. Migration and self-diffusion coefficients of divalent counterions in micellar solutions containing monovalent counterions were studied for solutions of Be2+ in lithium dodecyl sulfate and for solutions of Ca2+ in sodium dodecyl sulfate [193]. The structural disposition of the porphyrin complex and the conformation of the surfactant molecules inside the micellar cavity was studied by NMR on aqueous sodium dodecyl sulfate micelles [194]. 

The effects of ion valence and polyelectrolyte charge density showed that at very low ionic strength found that when the counterion valence of added salt changes from monovalent (NaCl) to divalent (MgS04), the reduced viscosity decreases by a factor of about 4.5. If La(N03)3 is used, the reduced viscosity will be further decreased although not drastically. As for polyelectrolyte charge density, the intrinsic viscosity was found to increase with it because of an enhanced intrachain electrostatic repulsion (Antonietti et al. 1997). 

Then, the ionic selectivity is discussed and related to the mechanism of crosslinking with divalent counterions. The sol-gel transition is then examined for LM and HM pectins and the mechanisms described in these two cases. The physical properties of the gels are related to the microstructure of the polymers and few data are examined. 

No specific ionic selectivity is really admitted in pectins with monovalent counterions due to the relativity low charge parameter a very interesting behaviour is observed when divalent counterions are considered. Specially, it was demonstrated that when DM<50% the activity coefficient of magnesium is much larger than that of calcium. The transport parameters (f) were found following the order [45] ... 

Some divalent cations such as Cu and Pb form very stable complexes with pectate, but are unlikely to be present at sufiScient concentration in the apoplast of plants to form a major fraction of the counterions associated with the pectic fraction in vivo. The Al ion may deserve closer examination, as it is certainly able to displace Ca from cell walls and reaches substantial concentrations in plant roots under some conditions [60,61]. aluminium is not usually considered to be freely translocated, however. Basic peptides with their negative charges spaced at a similar interval to galacturonans (0.43 nm or a small multiple thereof) can in principle have a very high afiBnity for pectate [62,63], but the extensins that are associated with the most insoluble pectic fractions [M] do not appear to have this type of structure. The possibility that the non-extractable pectic polymers in most cell walls are very strongly complexed with some cation other than Ca " cannot be ruled out, but there is little evidence to support it at present. 

Kwak J.C.T. Joshi Y.M. (1981) The binding of divalent metal ions to polyelectrolytes in mixed counterion systems. I. The Dye Spectrophotometric Method. Biophys. Chem. 13, 55-64. 

Figure 4.10 The effect of monovalent, divalent and trivalent counterions on the hydration state of neutralized poly(acrylic acid). Based on Ikegami (1964).

Although all divalent ions precipitate PAA when the degree of dissociation, a, approaches 10, there are differences when a = 0-25 (Figure 4.11). Small amounts of barium and calcium ions precipitate PAA at this low a value, whereas magnesium ions do not. These differences are not to be attributed to differences in the amounts of counterions bound, for condensation theory (Section 4.2.3) predicts that all divalent counterions are bound to polyanions to the same extent (Imai, 1961). Therefore, differences must arise from differences in solubility between the various polyacrylates. At low degrees of neutralization barium polyacrylate has low solubility, while magnesium polyacrylate is very soluble. This is related to the extent of disruption of hydration regions as cations are bound to polyions. 

Cations can be seen as acting as ionic crosslinks between polyanion chains. Although this may appear a naive concept, crosslinking can be seen as equivalent to attractions between polyions resulting from the fluctuation of the counterion distribution (Section 4.2.13). Moreover, it relates to the classical theory of gelation associated with Flory (1953). Divalent cations (Zn and Ca +) have the potential to link two polyanion chains. Of course, unlike covalent crosslinks, ionic links are easily broken and re-formed under stress there could therefore be chain slipping and this may explain the plastic nature of zinc polycarboxylate cement. 

The present coverage is divided according to the class of intermediate carbocations, anions, carbenes (and related divalent compounds) and radicals. Within each class, the usual set of models is assessed, the exception being that molecular mechanics models have been excluded. These have not been explicitly parameterized for charged species or molecules with unpaired electrons, and cannot be expected to perform favorably. Note that in some, but not all cases, the quality of the experimental structural data is not up to the same standard as for other small molecules. This is particularly true for carbocations and anions, where differences among counterions may lead to large differences in structure. 

Glasses exist that fnnction as selective electrodes for many different monovalent and some divalent cations. Alternatively, a hydrophobic membrane can be made semiper-meable if a hydrophobic molecnle called an ionophore that selectively binds an ion is dissolved in it. The selectivity of the membrane is determined by the structnre of the ionophore. Some ionophores are natnral products, such as gramicidin, which is highly specific for K+, whereas others such as crown ethers and cryptands are synthetic. Ions such as, 1, Br, and N03 can be detected using quaternary ammonium cationic surfactants as a lipid-soluble counterion. ISEs are generally sensitive in the 10 to 10 M range, but are not perfectly selective. The most typical membrane material used in ISEs is polyvinyl chloride plasticized with dialkylsebacate or other hydrophobic chemicals. 

With divalent counterions the two terminal carbanlons of the bolaform salts are associated with the same cation. The behavior of the n 2 salt again deviates from that of the other salts (31). In THF at 25°C the compound di(9-n-butyl fluorenyl)barlum (the two carbanlons are not connected by a chain) is a tight Ion pair while at -100°C the only stable species appears to be the mixed tight-loose Ion pair (reaction 11). 

It Is clear that the reactions with divalent cations are considerably more complex than those Involving monovalent cations, especially when ligands are added to promote Ion pair separation (32). Complex rate phenomena can be expected in anionic vinyl polymerization In the presence of divalent counterions. Some of these Interesting systems will be described elsewhere by other Investigators In this symposium. 

Divalent counterions Kinetic measurements using mono- and bifunctional initiators and Ba++ as the counterion in THF were reported by Mathis and Francois (37 ), who applied adiabatic calorimetry. At -7o°C no termination is found and conversion follows first order with respect to monomer concentration. The rate constants do not depend on the concentration of living ends, indicating the absence of free anions. The rate constants are smaller by a factor of 2o as compared with those measured with monovalent counterions. However, they are smaller by a factor of 3 only, compared with those calculated for chains which are intramolecular ly associated (Na+, counterion). The activation energy for PMMA Ba in THF is equal to that for monovalent counterions, but the frequency exponent is smaller by about 1.5 units, reflecting the fact that the transition state for the dianionic ion pair has higher steric requirements. 

The actual values of these concentrations depend on a whole array of unknown parameters, but their relative values depend only on the valence of the counterions. The entries outside parentheses in Table 13.1 are the values of the CCC relative to the value for the monovalent electrolyte in the same set of experiments. These are seen to be remarkably consistent for the divalent ions and acceptably close together for trivalent and tetravalent counterions. 

For a convenient example, D = D2, p2o = pio and N = 78pio, expression (4.4.42b) yields rj = 12, thus implying quite a selective extraction of divalent counterions from a mixture with monovalent ones, near the equilibrium. 

It has been known for more than 100 years that many aqueous dispersions precipitate upon addition of salt. Schulze and Hardy observed that most dispersions precipitate at concentrations of 25-150 mM of monovalent counterions [154,155]. For divalent ions they found far... 

Under environmental conditions, the solubility of anionic surfactants, especially of fatty acids, is affected remarkably by the presence of divalent cations. This is reflected in the dependence of the CMC on the counterion. Ca2+ and Mg2+ tend to precipitate anionic surfactants however, the solubility products of Ca(LAS)2 are relatively high (>1013 M 2, Matheson et al., 1985), so that precipitation of LAS is unlikely, given that LAS occurs in environmental waters usually at concentrations of less than lpM. 

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